This proposal describes experiments directed towards providing estimates of the dimensions of changes in gene expression during Drosophila development as well as the number and organization of these gene sequences in Drosophila genome. The standard technique of reassociation kinetics will be used to study both the number of different genes represented in mRNA populations at several stages of development in Drosophila and the amount of difference in the sets of genes expressed in the various developmental stages. The measurements obtained will give a minimum estimate of the number of different structural genes in the Drosophila genome as well as a measure of the amount of difference and overlap in the expression of these genes during development. The arrangement of the structural genes relative to one another and to the interspersed middle repetitive DNA sequences will be examined by electron microscopy. The position of the coding sequences and repetitive DNA sequences on very long single-stranded DNA fragments will be determined by labeling these sequences with two readily distinguishable electron dense markers. The results of the kinetic and microscopy studies will be used to evaluate the current one chromomere: one gene hypothesis as well as some of the features of proposed models for gene regulation.